Hot rolling method and hot rolling line

ABSTRACT

A hot rolling line and a hot rolling method capable of finer refining crystal grains are provided. Specifically, the hot rolling line for a metal sheet is composed of a finish rolling mill, a leveler, and a cooling apparatus installed from upstream to downstream in this sequence in a metal sheet transporting direction. The hot rolling method is such that, in the above hot rolling line, a metal sheet obtained by finish rolling a metal piece is subjected to repeated bending processing by the leveler and then cooled.

TECHNICAL FIELD

[0001] The present invention relates to a hot rolling method and a hot rolling line for manufacturing a high-strength metal sheet.

BACKGROUND ART

[0002] A hot rolling line for rolling a steel sheet will be described as a typical example of a metal sheet. The steel sheet is ordinarily manufactured by a hot rolling line schematically shown in FIG. 13.

[0003] A metal piece as a raw material is called a slab or a sheet bar. The slab may be heated in a not shown heating furnace and extracted therefrom or may be directly transported from an upper process without passing through the heating furnace. The sheet bar may be directly supplied to a finish rolling mill 3, and a steel sheet may be manufactured omitting the rolling thereof by a roughing mill 2. In FIG. 13, reference numeral 5 c and 5 d denote mandrels. The mandrels 5 c and 5 d are attached to coilers 5 a and 5 b, respectively and the rotation speeds thereof are controlled by a not shown controller. Each coiler winds a metal sheet 1 having been cooled by a cooling apparatus 4 around each mandrel and makes it to a coil-like metal sheet product. The slab or the sheet bar to be rolled is transported between the respective apparatuses by a multiplicity of not shown table rolls.

[0004] To increase the strength of the metal sheet product, various steel hot rolling methods have been examined to refine crystal grains. A typical method of them is a so-called controlled rolling method disclosed in Japanese Unexamined Patent Application Publication No. 63-223124, and the like. A principle of the controlled rolling method is to realize the refinement of crystal grains by increasing austenite (hereinafter, denoted by “γ”) grain boundaries where ferrite (hereinafter, denoted by “α”) nuclei are created in the transformation from γ to α and by introducing a lot of lattice defects such as dislocations, and the like to thereby create a lot of α grains in the transformation from γ to α. To refine the γ grains and to introduce the lattice defects such as the dislocations, it is effective to apply strain as large as possible to a steel sheet. However, since the thickness of the slab or the sheet bar and the thickness of a sheet product are predetermined, an amount of strain that can be introduced in an ordinary rolling process is limited. In general, it is said that the limit of an average grain size is 5 μm in the controlled rolling method.

[0005] Japanese Unexamined Patent Application Publication No. 60-44106 discloses that it is possible to obtain an ultra thin product having a uniform shape by disposing a tension application apparatus in the vicinity of the outlet side of the final stand of a finish hot rolling mill installed in a hot rolling line to which a high temperature raw material is connected.

[0006] Further, while the tension application apparatus disclosed in Japanese Unexamined Patent Application Publication No. 60-44106 has a multiplicity of bending rolls, it cannot apply a sufficient amount of bending strain to a finish-rolled metal sheet. This is because the tension application apparatus only applies tension to the finish-rolled metal sheet so that the ultra thin product having the uniform shape can be obtained.

[0007] The present invention is to propose a hot rolling method and a hot rolling line for realizing the refinement of the crystal grains of the structure of a metal sheet by applying strain thereto without changing the thickness of a slab or a sheet bar and the thickness of a product sheet and for more increasing the strength of the sheet product than that of a conventional sheet product. Further, the present invention is to propose a hot rolling method and a hot rolling line capable of refining crystal grains under the practically applicable condition of a leveler. Note that the term “metal sheet” used in the present invention also means a metal strip.

DISCLOSURE OF THE INVENTION

[0008] As a result of diligent study as to an increase in the strength of a metal sheet, the inventors have found a method of applying strain to the metal sheet without changing the thickness of a slab or a sheet bar and the thickness of a product. The inventors have found that it is effective to install a roller leveler (hereinafter, also simply referred to as leveler), in which upper and lower work rolls (hereinafter, also simply referred to as rolls) are disposed zigzag, behind a finish rolling mill and to apply repeated bending processing to a finish-rolled metal sheet. Since strain can be added to the metal sheet through bending deformation without reducing the thickness thereof, it is possible to add strain by repeating the bending deformation. Strain may not be uniformly added in the width direction of the metal sheet because the work rolls of the leveler are flexed by the force received from the metal sheet. To prevent this problem, the upper and lower work rolls may be backed up by back-up rolls, respectively. The present invention can be realized by adding it to a conventional hot rolling line by installing the leveler between the finish rolling mill and a coiler, thereby an equipment cost can be suppressed and the deterioration of productivity, and the like is not caused.

[0009] In the present invention, the leveler capable of adding repeated bending processing strain to the metal sheet is installed on the outlet side of the finish rolling mill. The application of the additional strain can refine γ grains (an increase in γ grain boundaries) in case of steel. In addition to the above, lattice defects such as dislocations into the γ grains are introduced, which make it possible to refine finer α grains. The strength of a product metal sheet is increased by refining the crystal grains of metal. Note that the metal sheet to which strain has been added by the leveler is further cooled to a desired temperature, and then wound. Further, the inventors have found at the same time that it is preferable to cool the metal sheet to a predetermined temperature by a cooling apparatus installed on the outlet side of the finish rolling mill and on the inlet side of the leveler, and then to further subject the metal sheet to the repeated bending processing.

[0010] The present invention is arranged as shown below.

[0011] 1. A hot rolling line for a metal sheet is characterized in that a finish rolling mill, a leveler for adding bending strain to the metal sheet, and a cooling apparatus are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.

[0012] 2. A hot rolling line for a metal sheet is characterized in that a finish rolling mill, a leveler in which work rolls are backed up by back-up rolls, respectively, and a cooling apparatus are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.

[0013] 3. A hot rolling line for a metal sheet is characterized in that a finish rolling mill, a cooling apparatus, a leveler, and an additional cooling apparatus are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.

[0014] 4. A hot rolling line for a metal sheet is characterized in that a finish rolling-mill, a cooling apparatus, a leveler in which work rolls are backed up by back-up rolls, respectively, and further a cooling apparatus are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.

[0015] 5. A hot rolling line for a metal sheet is characterized in that a joint apparatus, a finish rolling mill, a leveler for adding bending strain to the metal sheet, a cooling apparatus, a shearing apparatus, and a coiler are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.

[0016] 6. A hot rolling line for a metal sheet is characterized in that a joint apparatus, a finish rolling mill, a leveler in which work rolls are backed up by back-up rolls, respectively, a cooling apparatus, a shearing apparatus, and a coiler are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.

[0017] 7. A hot rolling line for a metal sheet is characterized in that a joint apparatus, a finish rolling mill, a cooling apparatus, a leveler, further a cooling apparatus, a shearing apparatus, and a coiler are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.

[0018] 8. A hot rolling line for a metal sheet is characterized in that a joint apparatus, a finish rolling mill, a cooling apparatus, a leveler in which work rolls are backed up by back-up rolls, respectively, an additional cooling apparatus, a shearing apparatus, and a coiler are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.

[0019] 9. A hot rolling line according to any of 1. to 8. is characterized in that the leveler has work rolls whose diameter is set to 300 mm or less.

[0020] 10. A hot rolling line according to any of 1. to 8. is characterized in that the leveler is a lever having 3 to 30 of work rolls.

[0021] 11. A hot rolling line according to any of 1. to 8. is characterized in that a drive system is employed in the respective work rolls of the leveler.

[0022] 12. A hot rolling line according to any of 1. to 8. is characterized in that guides are disposed in the intervals between the respective work rolls of the upper and lower work rolls.

[0023] 13. A hot rolling line according to any of 1. to 8. is characterized in that the surface roughness Ra of the work rolls of the leveler is set to 0.5<Ra<2.0 μm.

[0024] 14. A hot rolling line according to any of 1. to 8. is characterized in that the leveler has at least one small diameter work roll whose diameter is set to less than 40 times the thickness of the finish-rolled metal sheet.

[0025] 15. A hot rolling line according to any of 1. to 8. is characterized in that the leveler has at least one non-driven small diameter work roll whose diameter is set to less than 40 times the thickness of the finish-rolled metal sheet and the remaining work rolls have a diameter set to 40 times or more the thickness of the finish-rolled metal sheet as well as can be driven.

[0026] 16. A hot rolling line according to any of 1. to 8. is characterized in that the leveler has at least one non-driven small diameter work roll whose diameter is set to less than 40 times the thickness of the finish-rolled metal sheet, the small diameter work roll is arranged such that it has a back-up roll and drive torque is transmitted from the back-up roll to the small diameter work roll through the gears disposed to the respective necks of the small diameter work roll and the back-up roll, and the remaining work rolls have a diameter set to 40 times or more the thickness of the finish-rolled metal sheet and can be driven.

[0027] 17. A hot rolling method of subjecting a metal piece to hot rolling including finish rolling is characterized in that a metal sheet, which has been subjected to the finish rolling, is subjected to repeated bending processing by a leveler and then cooled.

[0028] 18. A hot rolling method according to 17 is characterized in that the metal sheet having been subjected to the finish rolling is cooled before the metal sheet is subjected to the repeated bending processing.

[0029] 19. A hot rolling method according to 17. or 18. is characterized in that the temperature of the metal sheet having been subjected to the repeated bending processing is set within the range from the Ar₃ point +50° C. to the Ar₃ point −100° C.

[0030] 20. A hot rolling method according to 17. or 18. is characterized in that the amount of push of the work rolls of the leveler is set to +1 to +30 mm.

[0031] 21. A hot rolling method according to 17. or 18. is characterized in that the temperature of the finish-rolled metal sheet is set to the Ar₃ point or higher.

[0032] 22. A hot rolling method according to 17. or 18. is characterized in that the transportation of the leading end and the tailing end of the metal sheet is tracked, and control is performed such that upper and lower work rolls are tightened after the leading end of the metal sheet has passed through the upper and lower rolls and the upper and lower rolls are opened before the tailing end thereof leaves the upper and lower rolls.

[0033] 23. A hot rolling method according to 17. or 18. is characterized in that the finish rolling is performed after a preceding sheet bar has been joined to a succeeding sheet bar.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1A is a view showing a layout of an example of a hot rolling line according to the present invention.

[0035]FIG. 1B is a view showing a layout of a preferable hot rolling line.

[0036]FIG. 2 is a schematic view explaining a leveler and a cooling apparatus applied to the present invention in detail.

[0037]FIG. 3A is a partly longitudinally sectional view showing a layout of rolls of an example of the leveler used in the present invention.

[0038]FIG. 3B is a schematic view showing a drive mechanism of large diameter work rolls adjacent to an upper small diameter work roll shown in FIG. 3A.

[0039]FIG. 3C is a view showing the influence of the ratio of the diameter of the work roll of the leveler to the thickness of a metal sheet on the average grain size of the metal sheet.

[0040]FIGS. 4A to 4C are views showing an arrangement of a drive mechanism for driving the small diameter work rolls of the leveler used in the present invention.

[0041]FIG. 5A is a schematic view explaining the inter-roll deformation of the metal sheet in the leveler.

[0042]FIG. 5B is a schematic view when the amount of push of the leveler is a negative amount.

[0043]FIG. 5C is a view showing a relationship between the diameter of the work rolls of the leveler and bending strain.

[0044]FIG. 6 is a schematic view showing a trouble caused when the leading end of the metal sheet passes through the leveler.

[0045]FIG. 7 is a schematic view showing a trouble caused by a metal sheet slip when it passes through the leveler.

[0046]FIG. 8 is a graph showing a relationship between the surface roughness of the work rolls of the leveler and a rate of occurrence of slip.

[0047]FIG. 9 is a schematic view showing how a measuring roll is installed to the hot rolling line.

[0048]FIG. 10 is a view showing an arrangement of the major ring roll.

[0049]FIG. 11 is a view showing a layout of another preferable hot rolling line according to the present invention.

[0050]FIG. 12 is a graph comparing the tensile strength and the average crystal grain size of the examples of the present invention with those of a conventional example.

[0051]FIG. 13 is a view showing a layout of a hot rolling line for manufacturing a conventional steel sheet.

BEST MODE FOR CARRYING OUT THE INVENTION

[0052] A hot rolling line according to the present invention will be explained using FIGS. 1A and 1B. In FIGS. 1A and 1B, a roughing mill 2, a first cooling apparatus 4, mandrels 5 c and 5 d, and coilers 5 a and 5 b are the same as those installed in the conventional hot rolling line shown in FIG. 13. Thus, they are denoted by the same reference numerals, and the explanation thereof is omitted.

[0053]FIG. 1A shows a first embodiment. In the hot rolling line, a finish rolling mill 3, a leveler 6, and a cooling apparatus 4 are installed from upstream to downstream in the rolling line in this sequence. Hereinafter, the cooling apparatus 4 is also referred to as a first cooling apparatus 4.

[0054]FIG. 1B shows a second embodiment. In the hot rolling line, a second cooling apparatus 7 is interposed between the finish rolling mill 3 and the leveler 6, in addition to the apparatuses of the hot rolling line shown in FIG. 1A. In the finish rolling mill 3, reference numeral 3 a denotes work rolls, reference numeral 3 b denotes back-up rolls, and these rolls are assembled in a not shown housing.

[0055] The leveler 6 includes at least three work rolls 6 a disposed zigzag and further back-up rolls 6 b for backing up the work rolls 6 a. It is preferable that the work rolls 6 a have a diameter set to 300 mm or less from the view point of adding lever-added-strain that will be described later. Further, when the diameter of the work rolls 6 a is less than 180 mm, it is preferable to provide the back-up rolls 6 b.

[0056] In the hot rolling line, a metal piece S is rolled through the finish rolling mill 3 and made to a metal sheet 1, and then the finish-rolled metal sheet 1 is subjected to repeated bending processing and then cooled. The crystal grains of the metal sheet product can be refined by the strain added to the metal sheet in the longitudinal direction thereof by the leveler 6.

[0057] In the case of steel, the repeated bending processing is applied to the metal sheet by the leveler 6 before it has transformed from γ to α, thereby α grains are refined. At the time, it is preferable to apply the repeated bending processing to the metal sheet in a two-phase temperature region in which an α phase slightly exists rather than applying the repeated bending processing to the metal sheet in a temperature region in which only a γ single phase exists because α grains can be finer refined thereby.

[0058] It is contemplated that a mechanism for refinement of crystal grains in the steel is created by a lot of α grains produced when transformation is performed from γ to α by that (1) γ grain boundaries are increased by refinement of γ grains, that (2) lattice defects such as dislocations, and the like are introduced into γ grains.

[0059] Note that, as shown in FIG. 1B, the second cooling apparatus 7 is interposed between the final stand of the finish rolling mill and the leveler, in addition to the first cooling apparatus 4. In the hot rolling line provided with the second cooling apparatus 7, the metal sheet 1 can be cooled to a desired temperature before the repeated bending processing is applied thereto after the metal sheet 1 has been finish rolled. This arrangement is preferable because the crystal grains of the metal sheet product can be finer refined.

[0060] The second cooling apparatus 7 can be arranged similarly to the conventional first cooling apparatus 4. The second cooling apparatus 7 is composed of, for example, cooling nozzles for ejecting cooling water to the front and back surfaces of the metal sheet 1, a controller for controlling the ejection of the cooling water, a radiation thermometer for measuring the surface temperature of the metal sheet 1, and the like. The temperature of the metal sheet 1 just before it is subjected to the repeated bending processing is preferably set to 900 to 750° C. when the metal sheet 1 is composed of steel.

[0061] As shown in FIG. 5A, bending strain ε at a time in the leveler 6 is proportional to δ/L² on the surface of the metal sheet 1 when the interval between the centers of the lower work rolls 6 a is shown by 2L and an amount of push of the rolls is shown by δ. Here, δ=0 shows a state in which the metal sheet is clamped by the upper work rolls and the lower work rolls. The distance in which the work rolls are pushed from this state is shown by “+”. It is known that the bending strain ε added to the surface of the metal sheet by the leveler 6 at a time is determined by the following formula (1).

ε=a×δ/L ²  (1)

[0062] where, a: 2×h, and

[0063] h: thickness of the metal sheet 1

[0064]FIG. 5C shows a relationship between the diameter d of the work rolls 6 a and the bending strain ε added at a time on the surface of the metal sheet. However, this is a result when the thickness of the metal sheet is set to 4 mm, the interval 2L between the centers of the work rolls 6 a is set to d+10 mm, and the amount of push 8 of rolls is set to a maximum amount of push. The bending strain ε added at a time on the surface of the metal sheet is inversely proportional to the diameter d of the work rolls 6 a. When the diameter d of the work rolls 6 a exceeds 300 mm, the bending strain ε added at a time on the surface of the metal sheet is greatly reduced. Accordingly, it is preferable that the diameter d of the work rolls 6 a be 300 mm or less. An increase in the diameter of the work rolls 6 a increases the interval 2L between the centers of the work rolls, thereby the length of the leveler is increased in the direction of the rolling line. The cooling length of the cooling apparatus also must be secured. Eventually, the length of the hot rolling line increases. Further, an increase in the diameter of the work rolls 6 a increases the apparatus in size. From the above point of view, it is preferable that the diameter d of the work rolls 6 a be 300 mm or less.

[0065] The strain added to the metal sheet 1 by the leveler 6 can be determined by multiplying the expression (1) by the number of times of bending (n−2), wherein n shows the number of the work rolls. When a work roll 6 a′ whose diameter is smaller than that of the other work rolls is disposed in the leveler 6 as shown in FIGS. 3A and 3B, strain added to the metal sheet 1 is increased because the radius of curvature of bending caused by the small diameter work roll 6 a′ is reduced. The inventors have found that a value stain near to an actual value of strain can be obtained by setting the value of “a” in the expression (1) to m×h. The value “m” is greater than 2 and up to about 3. The value “m” changes depending upon the conditions such as the amount of push δ of the leveler 6, tension acting on the metal sheet 1, and the like and can be determined by an experiment. The strain added to the surface of the metal sheet 1 when the small diameter work roll 6 a′ is provided can be determined from the following formula (2).

ε=h×δ×{2×n1/(L1)²+(m×n2)/(L2)²}  (2)

[0066] where, n1: the number of times of bending performed by large diameter work rolls,

[0067] n2: the number of times of bending performed by small diameter work roll,

[0068] L1: half the interval L2 between the centers of large diameter work rolls, and

[0069] L2: half the interval between the centers of two rolls 6 a facing the small diameter work roll across the metal sheet

n=n1+n2+2

[0070] Note that the large diameter work rolls mean the work rolls 6 a other than the small diameter work roll 6 a′.

[0071] Incidentally, when the amount of push δ of rolls is made excessively large to increase the longitudinal bending strain ε on the surface of the metal sheet 1, the leading end of the metal sheet 1 may not normally pass through the leveler as shown in FIG. 6. To prevent this phenomenon, it is preferable to restrict δ to +30 mm or less. In contract, it is preferable to add a minimum necessary amount of strain to the metal sheet and to set δ to at least +1 mm from the view point of refinement of crystal grains. Longitudinal bending strain ε can be increased on the surface of the metal sheet 1 by reducing the interval 2L between the work rolls 6 a by reducing the radius r thereof and maintaining the amount of push δ of the rolls. However, the work rolls 6 a are made slender and may be bent by the reactive force from the metal sheet 1 that is caused when the rolls are pushed. When the diameter of the work rolls 6 a is less than 180 mm, it is preferable to provide the back-up rolls for reinforcing the work rolls 6 a. The back-up rolls may be integrally arranged back-up rolls each composed of a roll member integrally extending in a roll axial direction or of a divided back-up roll composed of a plurality of roll members extending in a roll axial direction. However, the present invention is not limited thereto.

[0072] Further, it is also contemplated to add a desired amount of bending strain ε by adding a given amount of longitudinal bending strain ε on the surface of the metal sheet 1 by the leveler 6 each time and increasing the number of times of bending of the metal sheet 1. However, when the number of the work rolls exceeds 30, a problem arises in that the temperature of the metal sheet 1 is reduced and the reactive force from the metal sheet 1 is made excessively large. Thus, it is preferable to set the number of the work roll to 30 or less in the leveler installed in the hot rolling line of the present invention.

[0073] Further, it is preferable to individually drive the work rolls of the leveler. The employment of the work roll drive system makes it possible to repeatedly perform the bending processing while transporting the metal sheet. In principle, a driving speed is set equal to the traveling speed of the metal sheet. However, an exceptional speed is employed when the leading end or the tailing end of the metal sheet is located between the final stand of the finish rolling mill and the coiler. That is, a speed faster than the traveling speed of the metal sheet is set to the leading end thereof. A speed slower than the traveling speed of the metal sheet is set to the tailing end thereof. This is preferable to prevent the simultaneous travel of two or more metal sheets and a trouble caused thereby in the travel of the metal sheet. It is preferable to set the speed of the leading end to 103 to 140% of the traveling speed of the metal sheet. Further, it is preferable to set the speed of the tailing end to 60 to 95% of the traveling speed of the metal sheet while this speed changes depending upon the thickness and the traveling speed of the metal sheet.

[0074] It is preferable that the surface roughness Ra of the work rolls of the leveler applied to the present invention be 0.5<Ra<2.0 μm. This is to suppress a trouble caused when the metal sheet passes through the leveler as shown in FIG. 7. The surface roughness Ra means Ra (arithmetic average roughness) defined by JIS B 0601-1994 which is a value measured in the roll axial direction of the work roll by setting a cut-off value to 0.8 mm and an evaluation length to 4 mm.

[0075] A reason why the surface roughness Ra of the work rolls of the lever is set to 0.5 μm<Ra is as described below. When Ra is 0.5 μm or less, the metal sheet may not be able to pass through the leveler because of the slip arisen between the metal sheet and the work rolls of the leveler (refer to FIG. 7). A succeeding metal sheet is bent and overlapped with each other at the inlet of the leveler, thereby a trouble is arisen when it passes through the leveler. Thus, a rolling operation cannot help being interrupted. To suppress the occurrence of the trouble when the metal sheet passes through the leveler, slip in the leveler is prevented by setting the surface roughness Ra of the work rolls of the leveler to 0.5 μm<Ra so that the metal sheet can stably passes through the leveler.

[0076]FIG. 8 shows a relationship between the surface roughness Ra of the work rolls of the leveler and a rate of occurrence of slip (%). It can be found that when the surface roughness Ra of the work rolls of the leveler exceeds 0.5 am, the rate of occurrence (%) of slip is reduced in the leveler.

[0077] This investigation was performed under the same condition as that of the embodiment 12 (example 1 of the present invention) that will be described later using the hot rolling line shown in FIG. 1. Bending processing was performed repeatedly by changing the surface roughness Ra of the work rolls of the lever. The rate of the number of coils of metal sheets that caused a trouble by slip when the metal sheets passed through the leveler to the number of all the coils processed is set as the rate of occurrence of slip as to each surface roughness Ra (μm) of the work rolls. The surface roughness was measured at five points of each work roll in the axial direction thereof and the average value of the thus measured surface roughness at the five points was used as the surface roughness Ra (μm).

[0078] Note that the slip occurred often in the leveler when the work rolls of the leveler were pushed before the leading end of the metal sheet did not reach the pinch rolls of the coiler or the coiler after it had passed through the leveler.

[0079] A reason of this phenomenon can be presumed as described below. A force in a direction for moving the metal sheet forward without slip, which can be transmitted from the work rolls of the leveler to the metal sheet is shown by Fw, a push force, which is applied by the work rolls of the final stand of the finish rolling mill, is shown by Fc, and a force, which is necessary to cause the metal sheet to pass through the leveler, is shown by Fr. When the surface roughness Ra of the work rolls of the leveler is small, the repeated bending processing performed by the leveler without applying any tension to the leading end of the metal sheet at the outlet side of the leveler makes the sum of Fw and Fc smaller than Fr, which causes the metal sheet to slip in the leveler. In contrast, it is presumed that when the surface roughness Ra of the work rolls of the leveler is made coarse, Fw is increased and the sum of Fw and Fc is made larger than Fr, and thus the metal sheet does not slip in the leveler.

[0080] A reason why the surface roughness Ra of the work rolls of the leveler is set to Ra<2.0 μm resides in that if the surface roughness Ra of the work rolls of the leveler is set to 2.0 μm or more, the surface roughness of the work rolls is transcribed to the surface of the metal sheet to thereby make the surface thereof rough. In the case of a steel sheet, scales are partly exfoliated, and the quality of the surface is deteriorated.

[0081] In the present invention, the finish rolling mill, the leveler, and the cooling apparatus are installed from upstream to downstream in this sequence in the hot rolling line for the metal sheet. The metal piece having been finish rolled is made to the metal sheet, which is subjected to the repeated bending processing and then cooled. With this operation, the crystal grains are refined and a high-strength metal sheet product can be obtained. When the surface roughness Ra of the work rolls of the leveler is set to 0.5<Ra<2.0 μm, the occurrence of a trouble is reduced when the metal sheet passes through the leveler. This is preferable because the working ratio of the rolling line can be increased and the surface property of the metal sheet can be kept in a good state.

[0082] Incidentally, a problem may occur when the tailing end of the metal sheet passes through the leveler. The metal sheet may abruptly meander in a width direction while the tailing end thereof passes through the leveler, and thus the metal sheet may pass through the leveler in a corrugated and overlapped state. This phenomenon is called corrugation. There may be a case in which the work rolls of the leveler are scratched and the scratches thereof are transcribed to a succeeding metal sheet and deteriorate the surface quality thereof. To solve this problem, it is preferable to interpose threading guides 6 c between the work rolls 6 a and further to dispose side guides 6 d as a means, similarly to the problem of the leading end. The side guides 6 d are arranged such that guide plates are disposed in confrontation with each other on both the sides of the metal sheet 1 so as to clamp it in a width direction. Another means is to track the position of the tailing end of the metal sheet and to increase the intervals between the upper and lower work rolls of the leveler just before the tailing end enters the leveler.

[0083] The tracking will be explained here. The tracking means to sequentially detect the positions of the leading end and the tailing end of the metal sheet on the hot rolling line in real time. For example, a measuring roll 8 shown in FIG. 10 is installed on the inlet side of the finish rolling mill as shown in FIG. 9. The ratio of the thickness of the metal sheet on the inlet side of the finish rolling mill to the thickness thereof on the outlet side of the finish rolling mill is searched from the data stored in a not shown computer. The thickness ratio of the metal sheet is multiplied by the number of counted pulses generated from the measuring roll 8. With this operation, the position of the metal sheet on the hot rolling line can be found. Counting of the pulses is started when the leading end of the metal sheet is bitten between-the rolls of the final stand of the finish rolling mill.

[0084] The measuring roll 8 is rotated by being pressed against the metal sheet 1. The measuring roll 8 generates a pulse each time it rotates a predetermined angle (for example, 0.025 mm in a peripheral length). The speed of the metal sheet on the inlet side of the finish rolling mill is measured by counting the pulses generated by the measuring roll 8 by a not shown controller. Then, the speed of the metal sheet and the position of the leading end thereof on the outlet side of the finish rolling mill can be found in real time by multiplying the above speed by the ratio of the thickness of the metal sheet on the inlet side of the finish rolling mill to the thickness thereof on the outlet side of the finish rolling mill. The tracking of the tailing end of the metal sheet must be somewhat devised. The start point of the tracking is set to the timing at which the tailing end of the metal sheet leaves the rolls of the final stand of the finish rolling mill. The tracking is determined by integrating the product of the winding diameter of the metal sheet and the number of revolution of the mandrel in term of time from the time at which the metal sheet is wound around, for example, the coiler 5 a or 5 b. The winding diameter is calculated by adding the product of the number of windings and the thickness of the metal sheet to the diameter of the mandrel 5 c or 5 d. As another example, there is also a method of using the same start point, capturing the diameter and the cumulative number of revolution of the rolls of the final stand of the finish rolling mill by the not shown controller and searching a forward slip from the data stored in the not shown computer or calculating it from a model formula. Otherwise, a method of using a laser speed meter in place of the measuring roll is available. Any method may be applied as long as it is a method effective to embody the present invention in real time and capable of tracing the positions of the leading end and the tailing end. When the positions of the leading and tailing ends of the metal sheet 1 with respect the leveler can be determined in the not shown controller, it is sufficient only to control the rollers of the leveler so as to open and close them.

[0085]FIG. 11 shows a hot rolling line according to a third embodiment. A case in which the cooling apparatus 7 is not installed in front of the leveler 6 corresponds to an example (3) of the present invention, and a case in which the cooling apparatus 7 is installed in front of the leveler 6 corresponds to an example (4) of the present invention. A preceding sheet bar (preceding metal sheet 1 a) is joined to a succeeding sheet bar (succeeding metal sheet 1 b) by a joint apparatus 10. Next, after they have been rolled to the metal sheet 1 by the finish rolling mill 3, the metal sheet 1 is subjected to the repeated bending deformation. Subsequently, the metal sheet 1 is sheared by a shearing apparatus 16 and wound around the two coilers 5 a and 5 b separately.

[0086] A plurality of sheet bars are joined to each other by the joint apparatus 10. At a joined portion, a trouble caused to the metal sheet when it passes through the leveler 6 and the corrugation in the leveler 6 described above can be prevented in the leveler 6 as shown in FIG. 6. The yield of a high-strength metal sheet can be greatly improved as compared with a case in which sheet bars are rolled one by one and each metal sheet having a leading end and a tailing end is subjected to the repeated bending deformation.

[0087] While the joint apparatus 10 is composed a group of devices mainly including a coil box 11, a crop shear 9 a, and a joint device 12, a burr removing device 13, a joined portion cooling device 14, a sheet bar heating device 15, and the like, that are shown by dotted lines in FIG. 11, may be added thereto. Further, as a principle of joint, filler wire welding performed by laser, and the like have been put into practical use, in addition to a combination of induction heating and pressure welding. Any of the above methods may be employed, or a method other than them may be employed.

[0088] The tracking may be performed by counting pulses in the same way as that described above by assuming the leading end of a first joined metal sheet as the leading end described above and the tailing end of a finally joined metal sheet as the tailing end described above. Further, a method of using a laser speed meter in place of the measuring roll as described above may be employed. In short, any method may be employed as long as it can track the positions of the leading end and the tailing end in real time.

[0089] Sheet bars are joined to each other and continuously finish rolled, and further a resulting metal sheet is subjected to the repeated bending processing of the present invention. With this operation, the strength of the metal sheet can be greatly increased over the entire length thereof including the joined portions in the mid-flow thereof except the leading end of the steel sheet joined first and the tailing end of the steel sheet jointed finally.

[0090] As shown in FIG. 1B, the cooling apparatus 7 is installed between the outlet side of the finish rolling mill and the leveler. A preferable example is such that the finish-rolled metal sheet 1 is cooled until it is made to a predetermined temperature and then subjected to the repeated bending processing by the leveler.

[0091] In the case of steel, the outlet side temperature of the final stand of the finish rolling mill is ordinarily the Ar₃ point or higher to make the steel to a high-strength steel sheet. When the cooling apparatus 7 is installed on the inlet side of the lever, the steel sheet can be cooled in front of the leveler after it has been finish rolled. The refining effect of crystal grains can be maximized by setting the temperature on the outlet side of the leveler to the temperature range from the Ar₃ point, at which transformation from γ to α begins, or lower to the Ar₃ point −50° C. or higher.

[0092] The refining effect of the crystal grains is larger when bending strain is added a plurality of times to a metal structure in the midway of transformation in which almost all the portion of the structure is composed of γ grains and only a slight amount of α grains exists therein than when the bending strain is added a plurality of times in the temperature region exceeding the Ar₃ point. A larger action can be obtained from the former case which makes dislocations introduced into the γ grains to the nucleus creating sites of the α grains. It is contemplated that even if the bending strain is added a plurality of times after the completion of transformation from γ to α, the effect of refining the crystal grains is small.

[0093] The cooling apparatus 7 is composed of, for example, cooling nozzles 7 c for ejecting cooling water to the front and back surfaces of the metal sheet 1, a controller 7 b for control the ejection of the cooling water from the cooling nozzles 7 c, a radiation thermometer 7 a for measuring the temperature of the front surface of the metal sheet 1, and the like as shown by dotted lines in FIG. 2. The cooling apparatus 7 can cool the metal sheet 1 to a predetermined temperature according to the surface temperature thereof. The cooling apparatus 4 can be arranged similarly to the cooling apparatus 7. The metal sheet 1 can be cooled in a predetermined cooling pattern so that its temperature is made to a predetermined winding temperature. It is also possible to integrate the controllers 4 b and the controller 7 b as a single unit so that they can control temperature by sharing information. Reference numeral 4 a denotes a radiation thermometer installed in the vicinity of the outlet side of the cooling apparatus 4, and reference numeral 4 c denotes a cooling nozzles provided with the cooling apparatus 4. In FIG. 2, the leveler 6 is exaggeratedly drawn so that it can be easily understood.

[0094] In the leveler installed in the hot rolling line according to the first and second embodiments, at least one of the work rolls disposed therein is arranged as a small diameter work roll to thereby increase the strain added to the metal sheet 1. It is preferable not to drive the small diameter work roll and to permit the remaining large diameter work rolls to be driven (refer to FIGS. 3A and 3B). It is preferable to set the diameter d of the small diameter work roll 6 a′ to less than 40 times the thickness h of the finish-rolled metal sheet. It is preferable to set the diameter d of the large diameter work rolls 6 a to 40 times or more the thickness h of the finish-rolled metal sheet. The spindle yoke 61 a of a known universal joint 61 shown in FIG. 3B is fitted to the oval neck of each of the large diameter work rolls 6 a. The large diameter work rolls are driven in rotation by not shown motors through the universal joints 61. A gear box having a plurality of gears may be interposed between the motor and the universal joint 61.

[0095] Note that FIG. 3A is a schematic longitudinal sectional view of an example of the leveler installed in the hot rolling line according to the first and second embodiments. Further, FIG. 3B is a schematic view showing a drive mechanism of one upper small diameter work roll 6 a′ disposed in the leveler and two large diameter work rolls adjacent to it. The large diameter work rolls 6 a other than the above and the back-up rolls 6 b are omitted. In FIG. 3B, reference numeral 63 denotes bearings, and the work rolls 6 a and 6 a′ and the back-up rolls 6 b are rotatably supported by the frame of the not shown leveler through the bearings.

[0096] It is preferable that the leveler used in the present invention have at least one small diameter work roll 6 a′ whose diameter is less than 40 times the thickness h of the finish-rolled metal sheet. This is because that only the disposition of the at least one small diameter work roll having the diameter less than 40 times the thickness h of the finish-rolled metal sheet increases the bending strain ε added at a time by the small diameter work roll to thereby refine the crystal grains of a product. FIG. 3C shows the influence of the ratio d/h of the diameter of the work rolls of the leveler to the thickness h of the metal sheet on the average grain size of the metal sheet. The thickness of the steel sheet is 4 mm and 5 mm on the outlet side of the finish rolling mill. Hot rolling conditions are such that a finish outlet side temperature is 900° C., the steel sheet has a speed of 720 m/min on the outlet side of the finish rolling mill, and a winding temperature is 600° C. It can be found from FIG. 3C that the crystal grains of the product can be refined by setting the diameter d of the work rolls to less than 40 times the thickness h of the finish-rolled metal sheet. A reason why the diameter d of the work rolls is shown by d/h with respect to the thickness h of the metal sheet is that when the diameter d of the work rolls is reduced, the interval 2L between the rolls can be reduced and strain is added in reverse proportion to the reciprocal number of d/h.

[0097] It is preferable in the leveler used in the present invention to permit the large diameter work rolls 6 a to be driven and not to drive the small diameter work roll 6 a′. As described below, it is difficult to drive the small diameter roll 6 a′. The upper work rolls of the leveler must move upward and downward. To drive them by motors, universal joints, for example, are used. Since the small diameter work roll 6 a′ has the small diameter, a spindle yoke for a small diameter shaft is used in the universal joint. This spindle yoke cannot transmit a sufficient amount of torque. When a motor specified to cope with a large amount of drive torque is employed to forcibly transmit a large amount of toque, there is a possibility that the universal cannot endure the torque in its mechanical strength and is broken. It is difficult to directly drive the small diameter work roll 6 a′ using the universal joint as described above. In contrast, even if the work rolls having the large diameter that is 40 times or more the thickness of the finish-rolled metal sheet are directly driven through the universal joints, no problem is arisen in strength. Thus, it is preferable to drive only the large diameter work rolls 6 a so as to transmit the torque required to the bending processing of the metal sheet including the torque of the large diameter work rolls 6 a and the torque of the small diameter work roll that is not driven.

[0098] It is preferable that the leveler installed in the hot rolling line according to the present invention be composed of 11 work rolls or more to 30 work rolls or less and about one third the work rolls are arranged as the small diameter work rolls having the above diameter.

[0099] This is because that when the number of the not driven small diameter work rolls 6 a′ is up to four of the 11 work rolls disposed in the leveler, the torque, which is required to the not driven small diameter work rolls to subject the metal sheet to the bending processing, can be easily transmitted from the universal joints connected to the remaining seven large diameter work rolls without any problem in strength. In contrast, when the number of the not driven small diameter work rolls 6 a′ is five or more of the 11 work rolls disposed in the leveler, a problem in strength is arisen in the universal joints connected to the large diameter work rolls.

[0100] Further, when the number of the small diameter work rolls 6 a′ that are not driven is up to 10 of the 30 work rolls disposed in the leveler 6, the torque, which is required to the not driven small diameter work rolls to subject the metal sheet to the bending processing, can be easily transmitted from the universal joints 61 connected to the remaining 20 large diameter work rolls without any problem in strength. In contrast, when the number of the not driven small diameter work rolls 6 a′ is increased to 11 or more of the 30 work rolls disposed in the leveler 6, a problem in strength is arisen in the universal joints 61 connected to the large diameter work rolls 6 a. As described above, when the number of the small diameter work rolls is about one third or less the total number of the work rolls, the large diameter work rolls 6 a can be driven without any problem in the transmission of torque from the universal joints 61 for driving the work rolls 6 a, even if the small diameter work rolls 6 a′ are not driven.

[0101] It is advantageous that the large diameter work rolls of the leveler have a larger diameter from the view point of transmitting necessary torque. However, it is preferable to set the diameter of the work rolls 6 a to 300 mm or less from the view point of sufficiently adding strain to the metal sheet and reducing the size of the apparatus.

[0102] In the hot rolling line according to the first and second embodiments explained above, metal pieces are not connected to each other before they are finish rolled. As shown in FIG. 11, a hot rolling line according to a third embodiment is arranged such that a known joint apparatus 10 and a shearing apparatus 16 for shearing a continuous metal sheet 1 are installed in the hot rolling line according to the first and second embodiments. The hot rolling line of the third embodiment is arranged such that metal pieces S are finish rolled after they have been connected to each other and the continuous metal sheet 1 can be sheared while it travels.

[0103] The joint apparatus 10 in FIG. 11 is an apparatus for joining the tailing end of a preceding metal piece to the leading end of a succeeding metal piece. The joint apparatus 10 is mainly composed of a coil box 11, a crop shear 9 a, a joint device 12, and the like. The joint device 12 is composed of a joint unit using induction heating, laser, and the like. Further, a burr removing device 13, a joint portion cooling device 14, and a sheet bar heating device 15, which are shown by dotted lines, and the like may be added to the joint apparatus 10. Further, it is preferable to install the second cooling apparatus 7.

[0104] According to the hot rolling line of the third embodiment, it is possible to subject second and subsequent metal sheets joined to each other to the repeated bending processing by the leveler 6 over the entire length from the leading end thereof. As a result, the yield of a high-strength metal sheet is greatly improved. Thus, the hot rolling line according to the third embodiment is more preferable than that of the first and second embodiments in which the metal pieces S are rolled one by one.

[0105] Incidentally, in the leveler installed in the hot rolling lines according to the first, second, and third embodiments, when the small diameter work rolls 6 a′ are not driven as shown in FIGS. 3A and 3B, the metal sheet may slip between rolls in the repeated bending processing when the thickness thereof is increased. Thus, scratched defects due to slip may be caused on the metal sheet. In the leveler shown in, for example, FIGS. 3A and 3B, the inter-roll slip is arisen between the small diameter work rolls 6 a′ located on the metal sheet 1 and the back-up rolls 6 b disposed adjacent to the small diameter work rolls 6 a′ so as to reinforce them.

[0106] Thus, a mechanism shown in FIGS. 4a and 4B is contemplated to prevent the inter-roll slip in the leveler. Gears 64 and 64 a are disposed to the necks of the small diameter work roll 6 a′ and the back-up rolls 6 b, respectively. Drive torque is transmitted from the back-up rolls 6 b to the small diameter work rolls 6 a′ through the gears 64 and 64′. In the figures, reference numeral 64 denotes the gears fixed to the necks of the back-up rolls 6 b through keys, and the like, and reference numeral 64′ denotes the gears fixed to the necks of the small diameter work rolls 6 a′ through keys. In FIG. 4A, the load torque of the small diameter work rolls 6 a′ is transmitted in the following sequence. That is, the load torque is transmitted in the sequence of the gears 64′ fixed to the small diameter work roll 6 a′, the gears 64 fixed to the back-up rolls 6 b, the necks of the back-up rolls 6 b, the spindle yokes 61 a, the universal joints 61 provided with the spindle yokes 61 a, gear boxes 65 each having a plurality of gears, and the motor (not shown). The drive torque from the motor is transmitted in a sequence reverse to the above sequence.

[0107]FIG. 4A is a view showing an arrangement of a drive mechanism for driving the small diameter work rolls 6 a′ using the universal joints 61. This arrangement is preferable when the speed of the metal sheet 1 is low and about 300 m/min. FIG. 4A shows one small diameter work rolls 6 a′ disposed on an upper side and one back-up roll 6 b for reinforcing the small diameter work rolls 6 a′. The other work rolls are not shown in FIG. 4A. Reference numeral 62 in the figure denotes a bearing box to which bearings 63 for rotatably supporting the rolls are attached, and reference numeral 66 denotes a spindle support.

[0108] According to the drive mechanism of the small diameter work rolls 6 a′ of the leveler arranged as described above, drive torque can be transmitted from back-up roll 6 b to the small diameter work roll 6 a′ through the gears disposed to the neck of the small diameter work roll 6 a′ and the neck of the back-up roll 6 b. The remaining work rolls has the large diameter and can be driven. The inter-roll slip can be prevented by the above mechanism. The cost of the above arrangement is a little more expensive than that of the arrangement in which the small diameter work rolls 6 a′ are not driven.

[0109] In contrast, when the metal sheet 1 is passed through the leveler at a high speed of about 1000 m/min, there is an increased possibility that the universal joints are broken because of the drive limit thereof. To cope with this problem, a drive mechanism for the small diameter work roll 6 a′ arranged as shown in FIG. 4B is preferably. FIG. 4B shows an arrangement of several small diameter work rolls 6 a′ (four rolls in the figure) and back-up rolls 6 b for reinforcing them. The back-up rolls are directly coupled with the shafts of motors 67 and can endure a high speed rotation because no universal joint is used.

[0110] The small diameter work rolls are driven from the backup rolls through gears. Thus, not only the large diameter work rolls 6 a but also the small diameter work rolls 6 a′ can be driven. This drive mechanism is the same as the drive mechanism of the small diameter work roll 6 a′ shown in FIG. 4A except that the back-up rolls 6 b are directly coupled with the shafts of the motors 67. The explanation of a drive torque transmission path to the small diameter work rolls 6 a′ is omitted.

[0111] However, in the leveler shown in FIG. 4B, the drive motors 67, spindle supports 66, and the like are disposed on a lift plate 68 for integrally lifting the components relating to the upper work rolls 6 a′. The components relating to the upper work rolls 6 a′ are lifted by a not shown lift mechanism. When the components relating to the upper work rolls 6 a′ are arranged as the integral lift mechanism, no problem is arisen even if the diameter of the work rolls is set to 50 mm and the peripheral speed thereof is set to 1000 m/min, that is, the work rolls rotate at a high speed of 12700 rpm, thereby drive torque can be transmitted to the upper side small diameter work rolls 6 a′.

[0112] To further reduce the diameter of the work rolls 6 a′ a drive mechanism is arranged such that the components relating to the upper side work rolls 6 a′ is arranged as an integral lift mechanism as well as the motors 67 are directly and coaxially coupled with the upper side work rolls 6 a′ as shown in FIG. 4C. Note that no back-up rolls are shown in the figure. As can be found from FIG. 4C, since the diameter of the adjacent work rolls 6 a′ is too small at the time, the drive motors 67 are disposed on both the sides of the metal sheet 1 in a width direction across the hot rolling line. Even if this arrangement is employed, the drive motors 67 disposed on the same side mechanically interfere with each other in the space where they are installed. To prevent the interference, the lengths of the spindles of the drive motors disposed adjacent to each other on the same side are changed from each other. The diameter of the upper side small diameter work rolls can be reduced to 25 mm by the drive mechanism for the small diameter work rolls.

[0113] In the hot rolling lines explained above, the inventors executed various experiments to steels. As a result, the inventors have obtained new knowledge that a temperature at which the repeated bending processing is added by the leveler greatly affects the refining effect of α grains.

[0114] Materials used in the experiment were steels containing 0.2 C-0.7 Si-2.0 Mn-0.15 Ti. Each material was rolled to a thickness of 4 mm by the finish rolling mill. The number of the work rolls of the leveler was 23, the diameter of the work rolls was 190 mm, the interval between the center shafts of the work rolls was 200 mm, and an amount of push of the rolls was 20 mm. The steels were subjected to the repeated bending processing and then wound around the coiler. The Ar₃ point temperature of the steels was 750° C. The experiment was executed by adjusting the temperature of steel sheets at the outlet side of the leveler to 550 to 800° C. by variously changing a finish rolling speed and adjusting a cooling time, and the crystal grain size and the tensile strength of the experiment materials were actually measured.

[0115] By the way, the temperature of the steel sheets was measured by a not shown thermometer disposed at a position 1 m apart downstream from the most downstream roll of the leveler. As to the crystal grain size, the average sectional area of crystal grains was determined based on JIS G 0552 and an average grain size was calculated presuming that the average sectional area was a circle. The tensile strength was determined by cutting out No. 5 test pieces based on JIS Z 2201. No. 5 test pieces were cut out based on JIS Z 2201 by unwinding the steel sheets, which had been finish rolled and wound around coilers, at a different place. Note that the crystal grain size and the tensile strength were measured by cutting out measurement samples from the central portions of the steel sheets in the lengthwise direction of coils, that is, from the portions thereof which had been subjected to the repeated bending processing by the leveler.

[0116] Table 1 shows a result of the test and the measurement. In Table 1, an experiment material No. 1 shows a conventional example in which no leveler was used at the outlet side of the finish rolling mill. Experiment materials Nos. 2 to 7 show a result in which the repeated bending processing was performed by setting the temperature of the steel sheets to 800 to 550° C. at the outlet of the leveler and using the leveler so that the amount of push of the rolls was set to 20 mm.

[0117] As apparent from Table 1, crystal grains are refined in Nos. 2 to 5 which were processed using the leveler and whose temperature was 650° C. or more at the outlet side of the leveler as compared with the conventional example No. 1 which did not use the leveler. It can be found from the above fact that it is preferable to set the temperature of the metal sheets to the range from the Ar₃ point +50° C. to the Ar₃ point −100° C. on the outlet side of the leveler. In particular, the crystal grains of Nos. 3 and 4, in which the outlet side temperature of the leveler was set from the Ar₃ point to the Ar₃ point −50° C., were greatly refined.

[0118] In contrast, Nos. 6 and 7, in which the outlet side temperature of the leveler was 600° C. or less, correspond to a case in which they were subjected to the repeated bending processing after transformation was performed from γ to α because the deformation temperature in the leveler was low. It can be found that strain was added only to α grains and no crystal grains were refined.

[0119] A reason why the preferable temperature ranges exist can be presumed as described below. A metal structure that is being transformed in these temperature ranges has a structure in the midway of transformation in which almost all the portion of metal structure is composed of γ grains and α grains slightly exist therein. It is presumed that when deformation is applied to the metal structure, there can be obtained a large action for making dislocations introduced into the γ grains to the nucleus creating sites of the α grains as they are.

[0120] As apparent from what has been described above, it is possible to refine crystal grains by subjecting the finish-rolled metal sheets to the repeated bending processing by the leveler. The refining effect of the present invention can be maximized by cooling the finish-rolled metal sheet in front of the leveler and setting the outlet side temperature of the leveler from the Ar₃ point to the Ar₃ point −50° C.

[0121] To accurately control the temperature of the metal sheets on the outlet side of leveler, the tracking described above is performed and the metal sheets are properly cooled based on the temperature thereof actually measured on the outlet side of the finish rolling mill. It is preferable to perform the repeated bending processing by the leveler after the metal sheets are set to a predetermined temperature. It is possible to perform the repeated bending processing by the leveler while maintaining the metal sheets at an optimum temperature on the outlet side of the leveler from the temperature, the rolling speed, and the like of the metal sheets on the outlet side of the finish rolling mill.

EXAMPLE 1

[0122] As an example for verifying the effect of the present invention, experiment materials of two types of steels A and B shown in Table 2 were hot rolled, and a conventional example was compared with examples 1 and 2 of the present invention for study.

[0123] In the hot rolling, the outlet side temperature of the finish rolling mill was set to 900° C. and the experiment materials were finish rolled to a thickness of 4 mm under the condition of the speed of the steel sheets set to 720 m/min on the outlet side of the finish rolling mill, and the steel sheets were wound at 600° C. In the conventional example, after the steel sheets had been finish rolled in the hot rolling line, they were subjected to ordinary cooling and wound around coilers.

[0124] While the same conditions as those of the conventional example were basically employed in the example 1 of the present invention, steel sheets were subjected to the repeated bending processing by the leveler having 23 stages of work rolls after the steel sheets had been finish rolled. In the leveler, the work rolls had a diameter of 190 mm, the interval between the center shafts of the work rolls (the interval between upper rolls and the interval between the lower rolls) was 200 mm, and the amount of push of the rolls was 20 mm. Thereafter, the steel sheets were cooled and wound around coilers.

[0125] The center of the uppermost stream roll of the leveler was disposed at a position 30 m downstream from the center of the roll of the final stand of the finish rolling mill. The lengthwise surface strain added to the hot rolled steel sheets by the leveler was 0.34 approximation.

[0126] Conditions basically the same as those of the conventional example were employed in the example 2 of the present invention. However, steel sheets were cooled by the cooling apparatus additionally installed between the outlet side of the finish rolling mill and the inlet side of the leveler, and temperature control was performed so that the temperature of the steel sheets was set from the Ar₃ point to the Ar₃ point −50° C. on the outlet side of the leveler. The steel sheets were subjected to the repeated bending processing by the leveler, then cooled again, and wound around coilers. A plurality of banks were installed between the final stand of the finish rolling mill and the leveler as the cooling apparatus. The amount of cooling water was 3200 l/m² per unit surface area of the steel sheets on upper and lower sides thereof (that correspond to the front and back surfaces of the steel sheets) at the maximum. The temperature of the steel sheets was controlled from the Ar₃ point to the Ar₃ point −50° C. on the outlet side of the leveler while setting the number of the banks for ejecting cooling water to the finish-rolled steel sheets so as to remove the lengthwise partial temperature irregularity of the steel sheets on both the upper and lower surfaces thereof following the travel of the steel sheets.

[0127] Table 3 shows a comparison of the result of measurement of the crystal grain sizes and the tensile strengths of the conventional example with that of the examples 1 and 2 of the present invention. The positions where the measurement samples were cut out, the definition of the crystal grain size and the tensile strength, and a measuring method are the same as those described above.

[0128] In any of the steel types A and B, the strength of the example 1 of the present invention (Nos. 12 and 15) and the example 2 of the present invention (Nos. 13 and 16) that use the leveler is higher than that of the conventional example (Nos. 11 and 14) that do not use the leveler. Further, it can be found that the strength of the example 2 of the present invention (Nos. 13 and 16) to which the cooling apparatus has been applied is more higher than that of the example 1 of the present invention (Nos. 12 and 15) that used no cooling apparatus. FIG. 12 shows the comparison of the tensile strengths and the average crystal grain sizes of the experiment materials Nos. 11 to 13.

EXAMPLE 2

[0129] The hot rolling line shown in FIG. 1A, in which the finish rolling mill, the leveler and the first cooling apparatus were installed in this sequence from upstream to downstream, was used. Steel pieces were hot rolled to a thickness of 4 mm, and then cooled, and the average crystal grain sizes of ferrites and the tensile strengths of the hot rolled steel sheet products were examined.

[0130] The average crystal grain size was calculated by cutting out measurement samples from the central portions of the steel sheet products in lengthwise and width directions, determining the average sectional area of the crystal grains based on JIS G 0552, and presuming the sectional area as a circle. The tensile strength was determined by cutting out measurement samples from the central portions of the steel sheet products in a lengthwise direction, making No. 5 test pieces based on JIS Z 2201, and subjecting them to a tensile test at a room temperature.

[0131] Note that the steel sheets were arranged as Ti added steels whose component is shown in Table 4, the outlet side temperature of the final stand of the finish rolling mill was set to 900° C., the speed of the steel sheets was set to 720 m/min on the outlet side of the final stand of the finish rolling mill, and the winding temperature of the coiler was set to 600° C.

[0132] In the examples 1 to 6 of the present invention, the finish-rolled steel sheets were subjected to the repeated bending processing using a leveler having at least two small diameter work rolls whose diameter was set to less than 40 times the thickness of the finish-rolled metal sheets, and the state of the leveler was examined after it had been used, as shown in Table 5. Further, in the examples 1 to 6 of the present invention, the gear system shown in FIG. 4B was employed as the drive system of the small diameter work rolls 6 a′ of the leveler.

[0133] In the repeated bending processing, the intervals 2L between the center shafts between the work rolls on the upper and lower sides (between the upper work rolls and between the lower work rolls) were set to 155 mm in the examples 1 and 5 of the present invention (the diameter of the small diameter work rolls was 100 mm) and to 180 mm in the examples 2 to 4 of the present invention (the diameter of the small diameter work rolls was 150 mm) as well as the amount of push δ of the rolls was set to 20 mm, and leveler-added-strain was set to the values shown in Table 5. The lever-added-strain was calculated by setting m in the expression (2) to 3. The leveler was interrupted abruptly while it was in operation, and the curvatures of the steel sheets bent by the small diameter work rolls were measured. It was confirmed separately that the leveler-added-strain obtained from the experiment was accurately in agreement with the leveler-added-strain calculated using m=3. A decrease in the diameter of the work rolls (for example, comparison of the examples 1 and 2 of the present invention) and an increase in the number of the small diameter work rolls introduced (for example, comparison of the examples 3 and 4 of the present invention) increase the leveler-added-strain. Further, the Ar₃ point temperature of the steel sheets is as shown in Table 4. Rolling was performed such that the temperature of the steel sheets was set as shown in Table 5 on the inlet side of the leveler. The leveler was installed such that the center of the uppermost stream roll of the leveler is in agreement with a position 30 m downstream from the center of the final stand of the finish rolling mill.

[0134] In an example 6 of the present invention, the second cooling apparatus was installed in addition to the first cooling apparatus, finish rolled steel sheets were cooled by the second cooling apparatus before they were subjected to the repeated bending processing, and the temperature of the steel sheets was set as shown in Table 5 on the inlet side of the leveler. The other conditions were set similar to those the example 2 of the present invention.

[0135] A plurality of banks were installed between the final stand of the finish rolling mill and the leveler as the second cooling apparatus. The amount of cooling water was 3200 l/m2per unit surface area of the steel sheets on the upper and lower sides thereof (that correspond to the front and back surfaces of the steel sheets) at the maximum. The number of the banks for ejecting cooling water to the finish-rolled steel sheets was set so as to remove the lengthwise partial temperature irregularity of the steel sheets on both the upper and lower surfaces thereof following the travel of the steel sheets.

[0136] In a comparative example, finish-rolled steel sheets were subjected to the repeated bending processing using a leveler having only large disposed work rolls and setting the other conditions similar to those of the examples 1 to 6 of the present invention.

[0137] In contrast, as the conventional example, in the hot rolling line shown in FIG. 1A, steel pieces having the same component as that of the examples 1 to 6 of the present invention were used before the leveler was installed and finish rolled by setting the other conditions similar to those of the examples 1 to 6 of the present invention and then cooled.

[0138] Table 5 shows the average crystal grain sizes of ferrites and tensile strengths of the hot rolled steel sheet products of the resultant examples of the present invention, the comparative example and conventional example. Further, Table 5 also shows the states of the leveler after it has been used in the examples of the present invention and in the comparative example.

[0139] It can be found from the result of Table 5 that the crystal grains of the steel sheet products can be refined more in the examples 1 to 6 of the present invention that have been subjected to the repeated bending processing using the leveler having the small diameter work rolls than in the conventional example that has not been subjected to the repeated bending processing by the leveler.

[0140] Further, crystal grains can be refined more in the example 6 of the present invention in which the finished-rolled metal sheet has been cooled by the second cooling apparatus than in the example 2 of the present invention that has the same conditions as those of the example 6 of the present invention except that the steel sheet is not cooled by the second cooling apparatus before it is subjected to the repeated bending processing without installing the second cooling apparatus.

[0141] Further, it can be also found that, in the examples of the invention, the average grain sizes of the products can be refined by increasing the number of the small diameter work rolls and that the tensile strengths of the steel sheet products correspond to the crystal grain sizes and a steel sheet product having finer crystal grains has a larger strength.

[0142] Note that it is presumed that the a grains of the steel sheet produces of the examples 1 to 6 of the present invention and the comparative example are refined more than those of the conventional example because (1) γ grains are refined and γ grain boundaries are increased and (2) lattice defects such as dislocations into the γ grains are introduced by the repeated bending processing performed by the leveler.

INDUSTRIAL APPLICABILITY

[0143] The present invention makes it possible to increase the strength of a steel sheet than that of a conventional steel sheet. Further, since the mechanical characteristics of the steel sheet can be easily controlled without changing the component thereof, the present invention is useful from the view point of reducing a steel making and refining load and also has an energy saving effect. Further, the leveler having the small diameter work rolls can refine finer crystal grains than the leveler having only the large diameter work rolls, thereby a more strong product can be obtained. TABLE 1 Leveler output side Tensile No. temperature Grain size strength Reference 1 — 4.7 μm 670 MPa Conv. Ex. * 2 800° C. 2.6 μm 750 MPa Example 1 ** 3 750° C. 2.3 μm 780 MPa Example 1 4 700° C. 1.8 μm 820 MPa Example 1 5 650° C. 2.5 μm 750 MPa Example 1 6 600° C. 4.8 μm 700 MPa Comp. Ex. *** 7 550° C. 4.7 μm 710 MPa Comp. Ex.

[0144] TABLE 2 Type of steel Composition A Fe-0.2C-0.7Si-2.0Mn-0.15Ti B Fe-0.07C-1.8Si-1.5Mn-0.03Ti

[0145] TABLE 3 Type of Grain Tensile No. steel Leveler Cooler size strength Reference 11 A — — 4.7 μm 670 MPa Conv. Ex. 12 A ∘ — 2.6 μm 750 MPa Example 1 13 A ∘ ∘ 1.6 μm 830 MPa Example 2 14 B — — 10.9 μm  410 MPa Conv. Ex. 15 B ∘ — 8.4 μm 510 MPa Example 1 16 B ∘ ∘ 7.1 μm 550 MPa Example 2

[0146] TABLE 4 Type of Temperature at Ar₃ steel Composition point (° C.) A Fe0.2C-0.7Si-2.0Mn-0.15Ti 750

[0147] TABLE 5 Specification of leveler Leveler having small diameter Presence work rolls*¹⁾ Strip or Diameter d of temperature on State of Average Type absence small diameter Cooler leveler input Strain leveler grain Tensile of of work rolls Number of First Second side added by after it size strength No. steel leveler (nm) d/h rolls cooler cooler (° C.) leveler is used (μm) (MPa) Reference 1 A O 100 25   2 O — 850 0.38 O 2.2 780 Example 1 2 A O 150 37.5 2 O — 850 0.36 O 2.4 770 Example 2 3 A O 150 37.5 4 O — 850 0.39 O 2.0 790 Example 3 4 A O 150 37.5 8 O — 850 0.44 O 1.3 870 Example 4 5 A O 100 25   8 O — 850 0.53 O 1.0 910 Example 5 6 A O 150 37.5 2 O O 750 0.53 O 1.4 850 Example 6 7 A O Lever having work rolls other O — 850 0.34 O 2.6 750 Comp. Ex. than small diameter work rolls*²⁾ 8 A — — — — O — — — — 4.7 670 Conv. Ex. 

1. A hot rolling line for a metal sheet, characterized in that a finish rolling mill, a leveler for adding bending strain to the metal sheet, and a cooling apparatus are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.
 2. A hot rolling line for a metal sheet, characterized in that a finish rolling mill, a leveler in which work rolls are backed up by back-up rolls, respectively, and a cooling apparatus are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.
 3. A hot rolling line for a metal sheet, characterized in that a finish rolling mill, a cooling apparatus, a leveler, and an additional cooling apparatus, are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.
 4. A hot rolling line for a metal sheet, characterized in that a finish rolling mill, a cooling apparatus, a leveler in which work rolls are backed up by back-up rolls, respectively, and further a cooling apparatus are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.
 5. A hot rolling line for a metal sheet, characterized in that a joint apparatus, a finish rolling mill, a leveler for adding bending strain to the metal sheet, a cooling apparatus, a shearing apparatus, and a coiler are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.
 6. A hot rolling line for a metal sheet, characterized in that a joint apparatus, a finish rolling mill, a leveler in which work rolls are backed up by back-up rolls, respectively, a cooling apparatus, a shearing apparatus, and a coiler are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.
 7. A hot rolling line for a metal sheet, characterized in that a joint apparatus, a finish rolling mill, a cooling apparatus, a leveler, further a cooling apparatus, a shearing apparatus, and a coiler are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.
 8. A hot rolling line for a metal sheet, characterized in that a joint apparatus, a finish rolling mill, a cooling apparatus, a leveler in which work rolls are backed up by back-up rolls, respectively, an additional cooling apparatus, a shearing apparatus, and a coiler are installed in this sequence from upstream to downstream in the transporting direction of the metal sheet.
 9. A hot rolling line according to any of claims 1 to 8, characterized in that the leveler has work rolls whose diameter is set to 300 mm or less.
 10. A hot rolling line according to any of claims 1 to 8, characterized in that the leveler is a lever having 3 to 30 of work rolls.
 11. A hot rolling line according to any of claims 1 to 8, characterized in that a drive system is employed in the respective work rolls of the leveler.
 12. A hot rolling line according to any of claims 1 to 8, characterized in that guides are disposed in the intervals between the respective work rolls of the upper and lower work rolls of the leveler.
 13. A hot rolling line according to any of claims 1 to 8, characterized in that the surface roughness Ra of the work rolls of the leveler is set to 0.5<Ra<2.0 μm.
 14. A hot rolling line according to any of claims 1 to 8, characterized in that the leveler has at least one small diameter work roll whose diameter is set to less than 40 times the thickness of the finish-rolled metal sheet.
 15. A hot rolling line according to any of claims 1 to 8, characterized in that the leveler has at least one non-driven small diameter work roll whose diameter is set to less than 40 times the thickness of the finish-rolled metal sheet and the remaining work rolls have a diameter set to 40 times or more the thickness of the finish-rolled metal sheet as well as can be driven.
 16. A hot rolling line according to any of claims 1 to 8, characterized in that the leveler has at least one non-driven small diameter work roll whose diameter is set to less than 40 times the thickness of the finish-rolled metal sheet, the small diameter work roll is arranged such that it has a back-up roll and drive torque is transmitted from the back-up roll to the small diameter work roll through the gears disposed to the respective necks of the small diameter work roll and the back-up roll, and the remaining work rolls have a diameter set to 40 times or more the thickness of the finish-rolled metal sheet and can be driven.
 17. A hot rolling method of subjecting a metal piece to hot rolling including finish rolling, characterized in that a metal sheet, which has been subjected to the finish rolling, is subjected to repeated bending processing by a leveler and then cooled.
 18. A hot rolling method according to claim 17, characterized in that the metal sheet having been subjected to the finish rolling is cooled before the metal sheet is subjected to the repeated bending processing.
 19. A hot rolling method according to claim 17 or 18, characterized in that the temperature of the metal sheet having been subjected to the repeated bending processing is set within the range from the Ar₃ point +50° C. to the Ar₃ point −100° C.
 20. A hot rolling method according to claim 17 or 18, characterized in that the amount of push of the work rolls of the leveler is set to +1 to +30 mm.
 21. A hot rolling method according to claim 17 or 18, characterized in that the temperature of the finish-rolled metal sheet is set to the Ar₃ point or higher.
 22. A hot rolling method according to claim 17 or 18, characterized in that the transportation of the leading end and the tailing end of the metal sheet is tracked, and control is performed such that upper and lower work rolls are tightened after the leading end of the metal sheet has passed through the upper and lower rolls of the leveler and the upper and lower rolls are opened before the tailing end of the metal sheet leaves the upper and lower rolls.
 23. A hot rolling method according to claim 17 or 18, characterized in that the finish rolling is performed after a preceding sheet bar has been joined to a succeeding sheet bar. 